The present invention relates to a continuously variable speed transmission that can control its speed reduction ratio continuously, and more particularly to a continuously variable speed transmission that can responsively change a speed reduction ratio.
In using a continuously variable speed transmission, control of the speed reduction ratio is one of the important requirements in vehicle running performance.
Regarding the devices for controlling speed reduction ratio, as disclosed for instance in the Japanese Patent laid-open Publication No. 62(1987)-237164, there is known a control device for a continuously variable speed transmission comprising a constant displacement type hydraulic pump and a variable displacement type hydraulic motor, which device controls the speed reduction ratio by variably controlling the displacement of the hydraulic motor by way of a hydraulic actuator. In this connection, the hydraulic motor is a swash plate type axial piston motor, and the speed reduction ratio control is performed by controlling the slant angle of the motor swash plate with hydraulic servo units.
In performing a speed reduction ratio control of a continuously variable speed transmission with such an actuator as described above, the operational control of the actuator is effectuated, for example, by way of an actuation of solenoid valves controlled by a controller. The operational direction and speed of the actuator is often controlled in accordance with operational signals from the controller.
When a vehicle is running, the speed reduction ratio may sometimes be required to be decreased while it is in the operation of being increased, or vice versa. At such a time, with this control system, operational signals are output to the actuator from the controller to reverse the change direction of the speed reduction ratio. Accordingly the actuator attempts to reverse the change direction of the speed reduction ratio of the continuously variable speed transmission. However, during this operation the following difficulties may arise because of the following disadvantages of the prior-art control devices: possible play in a linkage connecting the actuator to the transmission or potential dead zones in the actuator may cause an invalid stroke of the actuator, thus resulting in time delay before the speed reduction ratio actually begins to change to the reversed direction after the command signals are inputted into the actuator, and eventually in an undesirable response in speed reduction ratio control.